1. Field of the Invention
The present invention relates, in general, to internal combustion engine control systems and, specifically, to an internal combustion engine control system capable of controlling NOx emissions.
2. Description of the Art
It is known in the art relating to internal combustion engines that operating an engine with a lean mixture of fuel and air improves efficiency of the engine. This means that for a given amount of work performed by the engine, less fuel will be consumed, resulting in improved fuel efficiency. It is also well known that reduction of NOx emissions when the fuel rate is lean is difficult to achieve, resulting in an almost universal use of stoichiometric operation for exhaust control of automotive engines. By operating an engine with a stoichiometric mixture of fuel and air, fuel efficiency is good and NOx emission levels are reduced by over 90% once the vehicle catalyst reaches operating temperatures.
In addition to air-to-fuel ratio, other parameters involved in the combustion process affect the level of NOx emitted from the engine. Exhaust gas recirculation, for example, is a process by which a portion of the exhaust gas produced from the engine is conducted back to the combustion chamber to reduce peak combustion temperatures. Because high combustion temperatures result in high NOx emissions, increasing the percentage of exhaust gas recirculated in the combustion process reduces NOx emissions. However, the level of exhaust gas recirculated into the combustion process is limited because excessive amounts would halt combustion.
Another parameter that affects NOx emissions from the engine is spark ignition timing in spark-ignition engines or, in the case of compression-ignition engines (diesels), fuel injection timing. The timing of spark ignition or fuel injection, whichever is applicable, in the combustion sequence affects both emissions and fuel consumption. To reduce NOx emissions, the engine""s timing is retarded from its baselinexe2x80x94the point of optimum fuel economyxe2x80x94lowering peak combustion temperature, increasing exhaust gas temperature, and increasing the amount of fuel burned. Since this decrease in NOx emissions occurs at the expense of increased fuel consumption, retard of an engine""s ignition or injection timing, whichever is appropriate, is subject to a maximum limit.
Because lean operation of the engine improves fuel efficiency, maximizing lean operating time is desirable. Catalysts and engine control technologies operate to minimize NOx emissions during lean operation of the engine. Two types of NOx catalyst devices are standard in the art. Most common is the NOx adsorber, which stores NOx emissions during fuel lean operations and allows release of the stored emissions during fuel rich conditions with conventional three-way catalysis to nitrogen and water. The other is the NOx converter, which continuously converts NOx emissions as the exhaust feedstream flows through the converter.
These NOx catalyst devices, however, are not 100% effective in reducing NOx emissions from vehicles under all operating conditions. Attempts to compensate for various operating conditions relied on controlling the amount of emissions in the feedstream through an indirect measure of catalyst efficiency. A signal from an oxygen sensor mounted after the catalyst controlled the air-to-fuel ratio in the cylinders. The presence of oxygen, as measured by the oxygen sensor, indicated the condition where NOx emissions were not being efficiently removed by the catalyst.
The catalyst was thus regenerated, or stoichiometric fueling was forced, both of which can lead to a loss in fuel efficiency.
Thus, it would be desirable to provide a means of directly controlling the NOx emissions in the feedstream to a level the catalyst can process efficiently under varying operating conditions to provide optimum emission control and improve fuel efficiency.
The present invention is a method for controlling NOx emissions in an internal combustion engine by monitoring the exhaust gases flowing from the engine to the NOx catalyst, detecting when this feedstream NOx is above the level of efficient processing by the NOx catalyst, and adjusting certain engine parameters to reduce the NOx level in the feedstream to a level the NOx catalyst can efficiently process. In this way, emissions of NOx are minimized while fuel efficiency is maximized.
Once the engine is operating, the engine-out NOx rate is calculated. Engine-out NOx rate is the level of NOx in the feedstream being processed by the NOx catalyst. The engine-out NOx rate can be modeled using known engine operating conditions or calculated from measurements obtained using existing sensor technology.
The control proceeds to calculate a maximum permissible engine-out NOx rate. The maximum permissible engine-out NOx rate is a function of NOx catalyst efficiency. NOx catalyst efficiency is a measure of the amount of NOx the catalyst can process, which is a function of certain characteristics of the catalyst feedstream, namely the temperature of the catalyst and the NOx level seen by the catalyst.
If the engine-out NOx rate is less than or equal to the maximum engine-out NOx rate, then the catalyst is not receiving more NOx than it can process. Since the engine is operating in a fuel efficient region, and NOx emissions are minimized, adjustments to engine parameters are not required. The control sequence then begins again with the determination of the engine-out NOx rate. When the engine-out NOx rate is greater than the maximum engine-out NOx rate, the catalyst is receiving more NOx than it can efficiently process. The control sequence then performs steps to increase at least one of the following engine parameters to reduce the engine-out NOx rate: exhaust gas recirculation; air-to-fuel ratio; and ignition or injection retard. Which order the adjustments are made in depends upon what type of NOx catalyst is in use, whether the engine is a spark-ignition or a compression-ignition engine, and the totality of engine operating conditions.
In one aspect of the present invention, under normal operating conditions in a spark-ignition engine where the NOx catalyst is in an adsorber, the exhaust gas recirculation (EGR) percentage is increased first, if possible, to reduce the engine-out NOx rate. The exhaust gas recirculation (EGR) percentage is compared to a pre-determined maximum EGR rate. If the percentage of EGR is less than the maximum EGR rate, the percentage of EGR is increased by a pre-determined increase factor according to conventional methods of increasing the amount of exhaust gas recirculated. The control sequence then begins again with the determination of the engine-out NOx rate at the engine""s new operating point.
If, however, the percentage of EGR is at its maximum EGR rate, then no changes in exhaust gas recirculated are made. The air-to-fuel ratio (A/F ratio) is compared to a pre-determined maximum A/F ratio. If the A/F ratio is less than a maximum A/F ratio, then the A/F ratio is increased by a pre-determined increase factor according to conventional methods of increasing cylinder fuel level. The control sequence then begins again with the determination of the engine-out NOx rate at the engine""s new operating point.
If, however, the A/F ratio is at its maximum A/F ratio, then the engine timing is compared to a pre-determined maximum value for engine timing. This maximum value for engine timing corresponds to the maximum allowable retard of an engine""s ignition or injection timing. In a spark-ignition engine, engine timing is spark ignition timing. In a compression-ignition engine, engine timing is fuel injection timing. If the engine timing is at the value indicating maximum retard, then the control sequence begins again with the determination of the engine-out NOx rate. If the engine timing is less than the value indicating maximum retard, then the retard indicated by the engine timing is increased by a calculated increase factor according to conventional methods of increasing retard in engine timing. The control sequence then begins again with the determination of the engine-out NOx rate at the engine""s new operating point.
The method for controlling NOx emissions of the present invention provides several unique features not found in previously devised methods for controlling NOx emissions. First, the method directly controls the level of engine-out NOx seen by the catalyst to a level that the catalyst can efficiently process, maximizing the amount of time the engine is able to operate lean.
Second, while methods of controlling exhaust gas recirculation and engine timing are well-known in the art, they have not been previously used to maximize lean engine operation as disclosed in the present invention.